The present invention relates generally to turbine power generating systems. More specifically, the present invention relates to a method and apparatus for purging fuel injector nozzles after combustion in the turbine combustor has ceased.
The United Sates Electric Power Research Institute (EPRI) which is the uniform research facility for domestic electric utilities, predicts that up to 40% of all new generation could be provided by distributed generators by the year 2006. In many parts of the world, the lack of electric infrastructure (transmission and distribution lines) will greatly expedite the commercialization of distributed generation technologies since central plants not only cost more per kilowatt, but also must have expensive infrastructure installed to deliver the product to the consumer.
Small multi-fuel, modular distributed microturbine generation units could help alleviate current afternoon "brownouts" and "blackouts" prevalent in many parts of the world. A simple, single moving part concept would allow for low technical skill maintenance and low overall cost would allow for widespread purchase in those parts of the world where capital is sparse. In addition, given the Unites States emphasis on electric deregulation and the world trend in this direction, consumers of electricity would have not only the right to choose the correct method of electric service, but also a new cost effective choice from which to choose. U.S. Pat. No. 4,754,607, which is assigned to the assignee of the present invention, discloses a microturbine power generating system suitable for cogeneration applications.
Liquid fuel units include a combustor and one or more fuel injector nozzles disposed within or leading into the combustion space of the combustor. Each nozzle has small orifices that cause the desired atomization of fuel upon injection into the combustor. Though these orifices are exposed to substantial heat during operation, fuel flowing through the nozzles provides cooling that prevents undesirable overheating of the nozzles. However, when operation of the unit is shut down the fuel no longer flows through the nozzles, which are exposed to residual combustor heat. If residual fuel is left in the nozzles, coking will occur. Being carbonaceous in nature, the residual fuel will undergo a destructive distillation reaction and form a coke-like or tarry residue on the nozzles. This residue will clog the nozzles and result in improper operation on subsequent startups.
The use of a stored volume of compressed air to purge fuel supply lines and/or nozzles is known. Offenlegungsschrift DE 3916477 discloses a compressor connected to a compressed air storage vessel. The vessel is also connected to the fuel supply line via a non-return valve. When the turbine is shut down by closing the fuel supply valve, pressure in the fuel supply line automatically falls permitting compressed air in the storage tank to automatically enter the fuel supply line and into the fuel injection nozzle so that any residual fuel in the fuel supply line or nozzle is blown into the combustion chamber and out through the turbine.
U.S. Pat. No. 4,041,695 to Harper et al., discloses a pneumatic purge system for gas turbine engines which includes an accumulator 82 for storing pressurized air at the maximum pressure developed by the compressor. In addition to the pressure accumulator tank, the purge system includes one-way check valves 80 and 90, and a solenoid shutoff valve 92. During normal engine operation the shutoff valve is in the closed position and the fuel valve 36 in the open position. Upon initiation of shutdown, the speed of the engine is first reduced to 60 to 70 percent of its maximum or optimum speed. Control 94 is then operated to close the solenoid fuel valve and simultaneously (or substantially simultaneously) open solenoid control valve 92. The resulting release of air purges all the remaining fuel from the fuel delivery system and into the combustion chamber of the engine while the combustion process is continuing. In this manner substantially all the fuel purged from the system is consumed in the combustion process, reducing emission of unburned hydrocarbons into the atmosphere. Continued air flow, after completion of the combustion process, blows off any residual traces of fuel off the spray nozzles which substantially eliminates carbon formation on such nozzles.
Though, at col. 3, ll. 57-59, Harper et al., indicates that engine 10 can be utilized as "an aircraft powering unit, auxiliary power unit or a stationary or other ground installation," the purging system is clearly designed for aircraft turbine engines. See, for instance, the drawing, col. 2, ll. 46-49 and col. 4, ll. 15-17. Further, while the system of Harper, et al. has the advantage of purging the fuel delivery system and substantially eliminating carbon deposits from the fuel nozzles, the process results in "a slight increase in the combustion process and in a slight, momentary acceleration of the engine." See col. 5, ll. 3-6. This may be acceptable when there is a controlled shutdown (i.e., the engine speed has already been reduced to between 60 to 70 percent of its maximum or optimum speed). However, a shutdown at maximum speed will result in an overspeed condition which may be excessive or otherwise undesirable. Overspeed by itself may result in mechanical failure of the rotating components. Also, immediate rapid shutdown at maximum speed may be required when an abnormal operating condition, such as a bearing or rotor failure is sensed, to prevent or minimize damage to turbine, compressor, generator or other system components.
Accordingly, it is an object of the present invention to shut down an engine without any increase in acceleration of the turbine (and, for that matter, any mechanically coupled components such as a compressor and a generator) and prevent carbon build up on the fuel injector nozzles.
It is a further object of the present invention to provide a purge system with a controller and associated sensors, in which the controller: (1) cuts off the flow of fuel to the combustor to cause deceleration of the turbine; and (2) only after combustion has ceased, causes the release of purging air to blow residual fuel off the fuel injection nozzles to minimize carbon buildup.